1. Field of the Invention
This invention relates to a multi-channel current measuring apparatus for applying a multichannel input signal current, obtained from an X-ray detector of an X-ray computerized tomograph, to each of a plurality of capacitors, and for reading the voltages of the capacitors, to examine the signal current in each channel.
2. Description of the Prior Art
An X-ray computerized tomograph is so constructed that an object to be examined is roetgenized and that the transmitted X-rays are detected by an X-ray detector, comprising a number of ionization chambers, to obtain projection data relating to the object based on signals provided by the detector. In this case, the tomograph is provided with a multichannel current measuring apparatus used to measure the strength of the output current of the ionization chamber in each of the multi-channels. To examine a number of ionization currents corresponding to the intensity of the transmitted X-rays thus obtained from the X-ray ionization chambers, a means may be used for applying each ionization current (hereinafter called input signal current) to a capacitor in each channel, once, and for measuring the strength of the charged voltage selectively at a subsequent time by the use of a sampling switch. However, disadvantageously, in such a method, simply using a sampling switch for reading the charged voltage of a capacitor may result in crosstalk, depending on distributed capacitance existing in the circuit.
In the multi-channel current measuring apparatus of the prior art, the ionization current, corresponding to the strength of the transmitted X-ray obtained from the X-ray ionization chamber, is converted into voltage on a per channel basis and digitalized by means of an analog-to-digital converter (AD converter). In this case, a leakage current may flow into the capacitor used for current/voltage conversion. In addition, because an amplifier for amplifying the charged voltage of a capacitor is not free from an off-set factor (also noise or other error factors) that cannot be ignored, it will be necessary to correct these error factors, if highly accurate measurement is required. Although the off-set factor of the amplifier may be corrected in an analog form through the so-called auto-zero method, no suitable correction means have been devised for reducing the error factors produced, for example, by the flow of leakage current in the circuit, into the capacitors.